In a semiconductor manufacturing apparatus, a washing process is performed using various types of chemical liquid after the wafer processing process. Many types of chemical liquid include a material with which bubbles are easily generated, such as surfactant. The chemical liquid is prepared by mixing chemical liquid and pure water in a preparation tank to attain a desired concentration. At this time, some chemical liquid may generate a large amount of bubbles.
The height of a liquid surface of the chemical liquid in the preparation tank is often measured by an ultrasonic sensor. In accordance with the result of measurement by the sensor, the volume of the chemical liquid and pure water to be supplied to the preparation tank is controlled.
However, if bubbles are generated in the preparation tank, the sensor may recognize the liquid level incorrectly. Specifically, when the ultrasonic wave ejected from the sensor is reflected from the bubbles, the reflected ultrasonic wave is received by the sensor in shorter time than from the actual liquid surface. The sensor, therefore, incorrectly recognizes that the liquid surface is higher than the actual liquid surface. Alternatively, the ultrasonic wave from the sensor may transmit through and be attenuated by the bubbles, and thus the sensor may be unable to receive the ultrasonic wave or may take very long time before receiving the ultrasonic wave. In this case, the sensor recognizes incorrectly that the preparation tank is empty.
It is, therefore, difficult to accurately measure the liquid surface of the chemical liquid in the preparation tank when bubbles are easily generated in the chemical liquid.
Similar problem may occur in a supply tank that stores the chemical liquid from the preparation tank and supplies the chemical liquid to a semiconductor manufacturing apparatus.